Displaying entity forms in Drupal 8

page.html.twig

<p>Please register</p>
{{ register_form }}

theme.theme

// We do this in the page preprocess function so 
// that we can access the variables from page.html.twig.
function theme_preprocess_page(&$variables) {
  // Since the register form is an entity form,
  // we need to create an entity associated with it
  $entity = \Drupal::entityManager()
    ->getStorage('user')
    ->create(array());
 
  // Also, an entity can have multiple forms, 
  // so we need to get the one for the register action.
  $formObject = \Drupal::entityManager()
    ->getFormObject('user', 'register')
    ->setEntity($entity);
  
  $form = \Drupal::formBuilder()->getForm($formObject);
  $variables['register_form'] = $form;
}